I am renovating my brother's 2nd floor bathroom in his 100+year old house. I have a lot of DIY experience, especially with tile work and I know that proper subflooring is essential to proper tile work, yet I haven't sistered joists before. The old floor had very small ceramic tiles but had high and low spots and had a fair amount of deflection. Taking up the tile (which was installed directly on to the plywood subfloor which I know is a no-no) The subfloor consisted of 2 layers of 3/4" plywood on top of the joists. The joists are 16"OC but they are the old 2x6's (2"x6") and they cover a 17' span, now I know where the deflection comes from! Here's the odd part, there is another set of joists, also 2x6 slightly lower than the floor joists, the 1st floor ceiling is attached to them. They are far enough horizontally from the floor joists to fit a modern 2x6 in next to the floor joists that I want to beef up. The bathroom is above a formal dining room, so there is no possibility of adding any type of support from below. Adding sisters to the entire 17' span is not an option, either. I am wondering what the best way would be to beef up the joists? I also want to set the sisters so that they are all level with each other, unlike the current floor. Adding 2x6's to one or even both sides of the joists in the bathroom area? Adding crossbracing? I already see from other sistering posts that using carriage bolts with large washers along with nails seems to be the method of attaching, but am I going to be able to add much stiffness here? Also, in an area where the toilet was, the joists were almost completely cut away for the toilet plumbing, I'll have to rebuild that area. Once the joists are in and level, I'll add two layers of plywood along with a CBU layer or Ditra for isolation under the tile. Any sistering advice is much appreciated! Also-how do I post pictures here-I could put up some pics of this stuff here.

 

I should add one more very basic question-If I can't sister the entire span of the joists, will it help much at all? I can only tear up the floor in the bathroom, which stretches from one exterior wall across about 9' of the 17' span. It still might make sense to add sisters to fix the floor leveling problem, but will it aid the deflection problem?

 

Yes, sistering just portions of the joist will strengthen the system. I like to glue and screw the joists together.

 

at the 9' wall intersection can you tie the new sister and maybe even the old joist to the plate of the wall. You could use some kind of TECO framing connector depending on the layout. Home Depot has a bunch of choices. This would definitely keep it from moving down. good luck

 

So if I follow right, you would have 3 sisters? The new wood between the two old? I think in this case I would consider sandwiching in plywood, doubled or whatever to fill the gap, bolt the whole mass together.

 

I think that makes the most sense, that way I could get stability from BOTH sets of joists.